This invention relates to a light receiving circuit used for repeaters and terminals, etc., in an optical transmission system.
Repeaters are used in optical transmission systems in order to transmit light signals to the remote terminals. A repeater converts the received light signal into an electrical signal and converts it again into a light signal after waveform equalization and shaping, and finally transmits it to the next repeater. In the terminal station, on the other hand, the received light signal is converted into an electrical signal and thereafter demodulation is carried out.
Automatic gain control (AGC) is carried out at repeaters and terminals so that an electrical signal of constant level can be obtained even if the level of the received light signal changes. FIG. 1 is an ordinary light receiving circuit having an automatic gain control circuit. In this FIG, 11 is a light signal; 12 is an avalanche photo diode (APD); 13 is an amplifier; 14 is a level detector; 15 is the output terminal; 16 is an AGC circuit.
The following methods are currently proposed for executing AGC using the circuit shown in FIG. 1.
(a) Only the multiplication factor M of APD 12 is controled. For example, when the light input level is doubled, M of the APD is reduced to a half (1/2). PA0 (b) The same as method (a) for low input levels with: M of the APD being fixed for high input levels and with the gain of amplifier 13 being controlled. PA0 (c) The same as method (a) for low input levels as is proposed in the published unexamined Japanese patent application No. 53-58748. The gain amplifier 13 is changed step by step for high input levels and simultaneously M of the APD 12 is also controlled. PA0 I.sub.o : APD output current for unit light input (when M=1) PA0 P: APD light input power PA0 M: APD multiplication factor PA0 N.sub.s : APD shot noise current power for unit light input (when M=1) PA0 x: Excessive noise figure of the APD PA0 Nth: Converted input noise current power of the next stage amplifier of APD
The methods listed above will contribute to providing an electrical signal of a constant level as an output even when the level of a received light signal changes. FIGS. 2 (1) to (3) show the relation between the light input level P, the multiplication factor M of the APD and the gain G of the amplifier in the methods (a) to (c) given above. In the same figures, the solid line indicates M, the broken line indicates G and the chained line indicates the optimum value of M, explained later. M.sub.min is the minimum value of the applicable multiplication factor of the APD. When M&lt;M.sub.min, which is not practical, the response speed of APD becomes very low due to an increase of the capacitance.
On the other hand, the signal to noise ratio (SNR) of the output signal of amplifier 13 is basically given by the following equation. EQU SNR=(I.sub.o PM).sup.2 /(N.sub.s PM.sup.2+x +Nth) (1)
Where,
The equation (1) teaches that SNR depends on P and M, and the optimum value M.sub.o for M gives the best SNR. From the equation (1), the dependency of M.sub.o on the light input level is expressed as follows. EQU M.sub.o =[(2/x) (N.sub.th /N.sub.sp)].sup./(2+x) ( 2)
Therefore, EQU M.sub.o .varies.P.sup.-1/(2+x) ( 3)
Usually, x is selected to be a value ranging from 0.3 to 1.
The gain G of the amplifier corresponding to M.sub.o is automatically determined from the condition that the output of the amplifier is maintained at a constant level and the determined gain is the optimum value. Here, since there is a difference between the M which keeps an electrical signal at a constant level and said optimum value M.sub.o, the SNR deteriorates if only an electrical signal level is kept at a constant level. FIG. 3 (1), (2) and (3) show such conditions. Namely, the figures show the relation between light input and SNR corresponding to the methods (a), (b) and (c). The solid line in the figure shows the ideal values, while the broken lines 32, 33, 34 respectively show the values of the methods (a), (b) and (c). Explained below is the reason why the solid line 31 and broken lines 32, 33, and 34 cross at the point P.sub.0, and the solid line 31 and the broken lines 33 and 34 cross at the point P.sub.3. As shown in FIG. 2 (1), (2), (3), the value of M crosses the optimum value M.sub.o at the points P.sub. 0 and P.sub.3. At the point P.sub.0, M becomes equal to M.sub.o (M=M.sub.o) by the setting, while at the point P.sub.3, M becomes also equal to M.sub.o. In a method being employed currently in order to improve the SNR, a value of M is not lowered to M.sub.min but instead is always kept at a value which is larger than than the value M.sub.1 (&gt;M.sub.min). Thereby, before the SNR deviates largely from the value SNR.sub.o for the condition M=M.sub.o, M is limited by M.sub.1 and the SNR is improved. This is shown in FIG. 4 with an example of the method (b). In the same figure, the solid lines 41, 44 and 47 show the ideal conditions, while the broken lines 42, 45, 48 show the conditions of the method (b), and the chained lines 43, 46, and 49 show the condition where M is larger than M.sub.1. However, the method where M is kept larger than M.sub.1 also has the following disadvantage. Namely, since the minimum value of M becomes M.sub.1 (&gt;M.sub.min), the additional gain variation width required is as much as (M.sub.1 /M.sub.min) in order to obtain the desired dynamic range. As can be understood from FIG. 4, when M&gt;M.sub.min, the amplifier gain width required only ranges from G1 to G2, but when M&gt;M.sub.1, it must range from G1 to G.sub.3 (G.sub.2 &gt;G.sub.3). As proposed in the unexamined Japanese patent application No. 53-90802, it is possible to simultaneously control the M of the APD and the gain of the amplifier in combination so that M of the APD always satisfies the equation (3) as shown in FIG. 2 (4) for variations of the light input level, but it has a disadvantage in that the control circuit is complicated.